The present invention relates to paving machines or pavers, and more particularly to pavers having a floating screed assembly.
A paver typically includes a chassis and a trailed (i.e., pulled from the rear) screed assembly which is articulated on the chassis by means of tension arms, commonly referred to as a "floating" screed. The "setting angle" of the screed relative to the ground (also referred to as the angle of attack) is capable of being adjusted, generally by means of actuating cylinders. Such screeds often include at least one tamper bar that is movable in upward and downward directions by means of a drive (e.g., a linkage driven by a rotating motor shaft) so as to have variable number of strokes. In other words, the drive is preferably adjustable so as to vary the stroke rate, the number of strokes per a unit of time, of the tamper bar. Further, the screed includes a bottom-side smoothing plate, also called a screed plate, which generally must have a "positive" setting angle when a mix of paving material is being paved. In other words, the front edge (i.e., "front" with respect to the direction of travel or paving direction) of the smoothing plate/screed plate is at a vertically higher position than the rear edge of the plate.
The desired setting angle or angle of attack of the smoothing plate depends on various parameters. These parameters include the amount of material compaction performed by the tamper, the amount of vibrational compaction if the smoothing plate has a vibration drive, the paving speed, the load-bearing capacity of the paving material, the weight of the screed and the desired thickness of the course or formed mat of paving material. Generally, tamper compaction and paving speed have the greatest effect on determining an appropriate setting angle of the smoothing plate.
An increase in tamper power leads to smaller setting angles of the screed, if the parameters otherwise remain unchanged. When the tamper power is increased, the tamper bar operates with a high amount of force, and combined with the screed weight and vibration (if any) acting on the screed, the following smoothing plate is capable of achieving only a limited amount of subsequent or "follow-up" compaction. The limit of the floating behavior of the screed is reached when the tamper bar is operated such that the bar substantially completely compacts the paving material. In that case, the following smoothing plate no longer has any effect on the paving material, so that the setting angle of the screed plate is therefore about 0.degree. (i.e., the plate is generally horizontal or level). Thus, the amount of tamper compaction must be reduced in order to operate the smoothing plate with a positive setting angle/angle of attack. Additionally, an increase in the paving speed also increases the setting angle.
Differences in the setting angle of the smoothing plate result in different degrees of compaction or compaction density in the formed mat of paving material. Although the finished course or mat of paving material preferably has an even or level surface, differences in the degree of compaction produced by the screed may result in an uneven mat profile being produced by subsequent rolling work. In other words, during subsequent rolling (e.g., with a compactor machine), the material settles to a greater extent at points on the material mat where lower precompaction by the screed has occurred than at points where the material has been pre-compacted to a greater extent.
It is known, for example from German Patent Application No. DE 40 40 029 C1, to measure the paving speed and to vary the frequency of the drive of a compacting assembly, particularly a tamper bar, according to a predetermined desired-value curve, in order to keep compaction generally equal at all points on the mat of paving material. However, on account of the dominant effect of tamper compaction, this method of controlling quality of paving operations is highly inaccurate as there is no proportional behavior between the compaction changes resulting from the paving speed change and the tamper frequency change.
Therefore, the object of the present invention is to provide a paver which allows more accurate compaction regulation and which accounts for the actual prevailing circumstances of the screed operation.